package javax.jms;

/** A client uses a <CODE>TopicSubscriber</CODE> object to receive messages that
 * have been published to a topic. A <CODE>TopicSubscriber</CODE> object is the
 * publish/subscribe form of a message consumer. A <CODE>MessageConsumer</CODE>
 * can be created by using <CODE>Session.createConsumer</CODE>. 
 *
 * <P>A <CODE>TopicSession</CODE> allows the creation of multiple 
 * <CODE>TopicSubscriber</CODE> objects per topic.  It will deliver each 
 * message for a topic to each
 * subscriber eligible to receive it. Each copy of the message
 * is treated as a completely separate message. Work done on one copy has
 * no effect on the others; acknowledging one does not acknowledge the
 * others; one message may be delivered immediately, while another waits
 * for its subscriber to process messages ahead of it.
 *
 * <P>Regular <CODE>TopicSubscriber</CODE> objects are not durable. They 
 * receive only messages that are published while they are active.
 *
 * <P>Messages filtered out by a subscriber's message selector will never 
 * be delivered to the subscriber. From the subscriber's perspective, they 
 * do not exist.
 *
 * <P>In some cases, a connection may both publish and subscribe to a topic.
 * The subscriber <CODE>NoLocal</CODE> attribute allows a subscriber to inhibit
 * the 
 * delivery of messages published by its own connection.
 *
 * <P>If a client needs to receive all the messages published on a topic, 
 * including the ones published while the subscriber is inactive, it uses 
 * a durable <CODE>TopicSubscriber</CODE>. The JMS provider retains a record of
 * this durable 
 * subscription and insures that all messages from the topic's publishers 
 * are retained until they are acknowledged by this durable 
 * subscriber or they have expired.
 *
 * <P>Sessions with durable subscribers must always provide the same client 
 * identifier. In addition, each client must specify a name that uniquely 
 * identifies (within client identifier) each durable subscription it creates.
 * Only one session at a time can have a <CODE>TopicSubscriber</CODE> for a 
 * particular durable subscription. 
 *
 * <P>A client can change an existing durable subscription by creating a 
 * durable <CODE>TopicSubscriber</CODE> with the same name and a new topic 
 * and/or message 
 * selector. Changing a durable subscription is equivalent to unsubscribing 
 * (deleting) the old one and creating a new one.
 *
 * <P>The <CODE>unsubscribe</CODE> method is used to delete a durable 
 * subscription. The <CODE>unsubscribe</CODE> method can be used at the 
 * <CODE>Session</CODE> or <CODE>TopicSession</CODE> level.
 * This method deletes the state being 
 * maintained on behalf of the subscriber by its provider.
 *
 * <P>Creating a <CODE>MessageConsumer</CODE> provides the same features as
 * creating a <CODE>TopicSubscriber</CODE>. To create a durable subscriber, 
 * use of <CODE>Session.CreateDurableSubscriber</CODE> is recommended. The 
 * <CODE>TopicSubscriber</CODE> is provided to support existing code.
 * 
 * @see         javax.jms.Session#createConsumer
 * @see         javax.jms.Session#createDurableSubscriber
 * @see         javax.jms.TopicSession
 * @see         javax.jms.TopicSession#createSubscriber
 * @see         javax.jms.MessageConsumer
 */

public interface TopicSubscriber extends MessageConsumer
{

   /** Gets the <CODE>Topic</CODE> associated with this subscriber.
    *  
    * @return this subscriber's <CODE>Topic</CODE>
    *  
    * @exception JMSException if the JMS provider fails to get the topic for
    *                         this topic subscriber
    *                         due to some internal error.
    */

   Topic getTopic() throws JMSException;

   /** Gets the <CODE>NoLocal</CODE> attribute for this subscriber. 
    * The default value for this attribute is false.
    *  
    * @return true if locally published messages are being inhibited
    *  
    * @exception JMSException if the JMS provider fails to get the
    *                         <CODE>NoLocal</CODE> attribute for
    *                         this topic subscriber
    *                         due to some internal error.
    */

   boolean getNoLocal() throws JMSException;
}
